Ammonites are named after the Egyptian god Ammon, who is often depicted with rams' horns behind each ear.

Female ammonites grew up to 400 percent larger than males, presumably to make room to lay eggs.

Ammonites were predatory, squidlike creatures that lived inside coil-shaped shells. Like other cephalopods, ammonites had sharp, beaklike jaws inside a ring of tentacles that extended from their shells to snare prey such as small fish and crustaceans. Some ammonites grew more than three feet (one meter) across—possible snack food for the giant mosasaur Tylosaurus.

Ammonites constantly built new shell as they grew, but only lived in the outer chamber. They scooted through the warm, shallow seas by squirting jets of water from their bodies. A thin, tubelike structure called a siphuncle reached into the interior chambers to pump and siphon air and helped them move through the water.

Ammonites first appeared about 240 million years ago, though they descended from straight-shelled cephalopods called bacrites that date back to the Devonian, about 415 million years ago. Ammonites were prolific breeders, lived in schools, and are among the most abundant fossils found today. They went extinct with the dinosaurs 65 million years ago. Scientists use the various shapes and sizes of ammonite shells that appeared and disappeared through the ages to date other fossils.

Fibonacci Sequence

[EDIT: And what's really interesting to me about this 240 million year old snail/squid monster, is that it's another awesome natural occurrence of the Fibonacci sequence. To me, another glimpse of the fascinating math at the root of all life. Another amazing repeatable mathematical pattern evident throughout the chaos of random evolving life. Natural beauty, definable by math and science.]

The shape of the ammonite shell (and the shell of all nautiloids) are aesthetically pleasing because they are one of the natural expressions of the Fibonacci spiral, or golden spiral (also observed in galaxies, the unfurling of fern fiddleheads, the arrangement of leaves around a stem, the fruitlets of a pineapple, the flowering of artichoke, and the arrangement of a pinecone, just to name a few...). A "golden spiral" is easy to construct: 1**2 + 1**2 + . . . + F(n)**2 = F(n) x F(n+1)

The golden spiral is a symbol of beauty and proportional perfection/fit with unlimited room for expansion. The logarithmic proportions of the spiral are consistent throughout, no matter how large the spiral becomes, and the unique spiral can be found throughout the human body and in nature.

The spiral follows a specific mathematical formula: Fn = Fn-1 + Fn-2. Each number in the sequence is the sum of the previous two. If you construct a series of squares with lengths equal to the Fibonacci numbers (1,1,2,3,5, etc) and trace a line through the diagonals of each square, it forms a Fibonacci spiral.